The present invention relates to an MRI apparatus to which a high-resistance magnet is applied and a production method of the high-resistance magnet.
In a conventional MRI apparatus in which a ferromagnetic material is used, magnetic field gradient combines with a shield and an electrically conductive structure in a magnet and thereby eddy current is generated as described in JP-A No. 166898/2000. Because of the eddy current, the increase and attenuation of magnetic field gradient are observed during and after the application of a trapezoidal current pulse to a gradient coil and the phenomenon leads to image distortion.
In the conventional invention as stated above, a magnetic material that is likely to cause eddy current is used. The eddy current is one of the causes of deviation from an ideal magnetic field gradient waveform and causes image distortion, loss of strength, ghost generation, loss of signal, and spectral distortion. In order to suppress eddy current and reduce hysterisis that causes loss, a high-resistance layer is applied to parts of a permanent magnet and a soft magnetic material, thus the high-resistance layer reduces eddy current flowing among particles in those members and enhances the heat resistance thereof, and hysterisis is reduced by heat treatment and the like.